The present invention relates to a high performance X-ray generating target. More particularly, the invention is directed to a method of making a high performance rotating X-ray tube anode structure having an improved target face.
X-rays are produced when electrons are released in a vacuum within an X-ray tube, accelerated and then abruptly stopped. The electrons are initially released from a heated, incandescent filament. A high voltage between an anode and cathode accelerates the electrons and causes them to impinge upon the anode. The anode, usually referred to as the target, can be a rotating disc type so that the electron beam constantly strikes a different point on the target surface. The X-ray tube contains the cathode and anode assembly, which includes the rotating disk target and a rotor that is part of a motor assembly that spins the target. A stator is provided outside the X-ray tube vacuum envelope, overlapping about two-thirds of the rotor. The X-ray tube is enclosed in a protective casing having a window for the X-rays that are generated to escape the tube. The casing is filled with oil to absorb heat produced by the X-rays.
The rotating X-ray tube target includes a refractory metal target substrate and a target focal track of an X-ray emitting metal joined to the target substrate along an interface. Tungsten alone and tungsten alloyed with other metals are commonly used in X-ray targets. Metals, which are sometimes alloyed with the tungsten in small amounts, include rhenium, osmium, irridium, platinum, technetium, ruthenium, rhodium and palladium. X-ray targets formed wholly from tungsten or from tungsten alloys where tungsten is the predominant metal are characterized by high density and weight. Additionally, tungsten is notch sensitive and extremely brittle and is thereby subject to catastrophic failure. Because of these shortcomings, X-ray targets typically comprise a tungsten or tungsten alloy target focal track and a target substrate of another metal or alloy. Typically, molybdenum and molybdenum alloy are used for the target substrate.
An X-ray target is typically formed by a powder metallurgy technique wherein metal powder to form the target focal track is placed against metal powder to form the target substrate. The resulting powder mass is pressed, sintered and then forged and machined to form the target.
The process results in an uneven surface and thickness and an uneven interface between target focal track and target substrate metal. The focal track metal is relatively heavier than the target substrate metal and the uneven thickness can cause an imbalance in the rotating target. Thin and thick areas in the track produce stress at the track/target interface that can cause localized grain growth and delamination. The inability to accurately control the thickness of the track metal requires that an excess of expensive track metal be applied to the target substrate to assure that no target substrate metal is left exposed. There is a need for a process to apply a focal track with both n even surface and an even interface between track and X-ray target substrate.
The invention provides an improved X-ray tube rotary anode. The anode comprises a refractory metal target substrate and a refractory metal focal track formed on the substrate with a focal track/target substrate interface that varies less than about xc2x10.13 mm from a perfect plane interface between track and target substrate.
In an embodiment, the invention relates to an X-ray tube having a rotating anode assembly comprising a refractory metal target substrate and a refractory metal focal track formed on the substrate with a focal track/target substrate interface that varies less than about xc2x10.13 mm from a perfect plane interface between track and target substrate.
In another embodiment, the invention relates to a process of making an X-ray target. In the process, a refractory metal is deposited onto a surface to form a tape. The X-ray target is formed from the tape applied to a refractory metal substrate.
In another embodiment, the invention relates to a process of forming an X-ray target, comprising casting a slurry of a powder in a solvent containing a binder onto a casting surface, evaporating the solvent from the slurry to produce a tape of a shaped layer removably adhering to the casting surface, densifying the tape to increase its green strength, peeling the casting surface from the shaped layer, forming a pack from the shaped layer and an X-ray target substrate-forming material and sintering the pack to produce the X-ray target.